This invention relates to a VIR-controlled hue correction circuit for use in video display apparatus and, more particularly, to such a hue correction circuit which produces a correction signal that is a function of the luminance level of the received composite color video signal.
In video display apparatus, such as a color television receiver, phase shifts in the chrominance signal of the composite color video signal supplied to the video display apparatus can result in hue distortion in the reproduced color picture. That is, the hue of the reproduced color picture will differ from that of the originally televised scene. If the composite color video signal which is supplied to the video display apparatus is a transmitted signal, such phase shifts may be attributed to various disturbances in the transmission path which affect the chrominance signal to a different degree than the burst signal. Because of this, the chrominance signal may be shifted by an amount which differs from a phase shift in the burst signal, thereby preventing the usual control circuits from eliminating any relative phase differential between the chrominance and burst signals.
If the video display apparatus is a television monitor for displaying video pictures reproduced from a video recorder, such as a video tape recorder (VTR), a phase shift may be imparted to the chrominance signal, resulting in a differential phase error between the chrominance and burst signals, with a concomitant hue distortion in the reproduced video picture. This differential phase error is caused by the fact that, in many VTR devices, the chrominance signal is frequency-converted to a lower frequency range while the luminance signal frequency modulates a higher frequency carrier, the frequency-modulated luminance signal then serving as a carrier onto which the frequency-converted chrominance signal is superimposed. As a result thereof, the frequency-modulated luminance component serves as a biasing level for the frequency-converted chrominance component. Consequently, upon reproduction, the chrominance signal is subjected to phase distortion in dependence upon changes in the luminance signal.
In order to minimize the aforementioned differential phase error due to factors in the transmission path or due to the changing biasing level of the chrominance signal in a video recorder, it has been proposed to insert a vertical interval reference (VIR) signal into a predetermined portion of the vertical interval of the video signal. For the purpose of standardization, the VIR signal is inserted into the nineteenth line interval of the transmission of many broadcasted color video signals. The purpose of this inserted VIR signal is to provide certain predetermined reference parameters, or characteristics, whereby video signal receiving apparatus, upon detecting these parameters or chracteristics, can effect an automatic hue correction operation, whereby the ultimately reproduced color video picture is substantially free of hue distortion.
The VIR signal which is used in television broadcast transmissions is formed of a reference subcarrier, equal in frequency and phase to the usual color burst signal, this reference subcarrier being superposed on a predetermined reference luminance level and being transmitted during a preselected chrominance reference portion of the VIR signal. This is designated as the chrominance reference component. Following this chrominance reference component, the VIR signal is provided with a luminance reference component of predetermined amplitude and duration. A black reference component of a respectively predetermined amplitude and duration then follows the luminance reference component. When the VIR signal is received, or reproduced from a video recording, the reference information provided by the chrominance reference component, the luminance reference component and the black reference component is used to control various characteristics, such as the gain or level of the chrominance channel, thereby controlling the color saturation of the reproduced video picture, and the phase of the locally generated oscillating signal used for demodulation, thereby controlling the hue characteristic of the reproduced picture. Although the chrominance and luminance reference components included in the VIR signal would be visible, it is appreciated that the nineteenth line interval is at a portion of the raster scanned by the electron beam in the CRT which is not viewed. Hence, the VIR signal does not interfere with the displayed video picture.
In previously proposed VIR-controlled hue correction circuits, the level of one of the demodulated color difference signals produced in response to the VIR signal is detected. If this demodulated level differs from a predetermined level, this difference corresponds to a differential phase error between the phase of the local oscillating signal which is used for demodulation and the received chrominance reference component. Consequently, the resultant phase error is used to vary the phase of the local oscillating signal, thereby minimizing the hue distortion. Typical of this type of VIR-controlled hue correction circuit is described in U.S. Pat. No. 3,950,780, issued Apr. 13, 1976. An improved VIR-controlled hue correction circuit is disclosed in our copending application No. 839,847, filed Oct. 6, 1977. Yet another example of such VIR-controlled hue correction circuitry is disclosed in our copending application No. 825,186, filed Aug. 16, 1977.
In the VIR signal which currently is used in broadcast transmissions, the chrominance reference component is superposed onto a luminance reference level of 70 IRE units. This luminance reference level corresponds to the average luminance level of Caucasian skin tones. It is expected that, in a displayed video picture, although the hue of various objects therein may be unknown, and may not be recognized, the hue of Caucasian skin tones will be readily perceived and identified. Thus, the standardized VIR signal is intended to correct the hue of the displayed video picture so that such Caucasian skin tones are reproduced correctly.
In many instances, the hue of the reproduced video picture varies as a function of the luminance level. Thus, although the correct hue is displayed for Caucasian skin tones, other luminance levels, and particularly other skin tones, such as Asian or Mongolian skin tones, Negro skin tones, and the like, may not be reproduced accurately. That is, since the chrominance reference component is superimposed onto the luminance level of 70 IRE units, other chrominance signals which are superposed onto different luminance levels may have differential phase errors which are not fully corrected. Thus, the skin tones of Mongolian or Asian skin, having luminance levels between 40-60 IRE units, and skin tones for Negro skin having luminance levels between 20-40 IRE units may be reproduced with improper hue. Hence, hue distortion may be present in video pictures of images of persons having skin tones other than Caucasian. Hue distortion also will be present in the images of objects whose luminance levels differ from the average Caucasian level of 70 IRE units.
In our copending application Ser. No. 894,513, filed Apr. 7, 1978, VIR-controlled hue correction circuitry is disclosed for overcoming the aforementioned problems. As disclosed therein, the differential phase error between the burst signal and the chrominance reference component of the VIR signal is controlled in dependence upon the luminance level of the received video signal. The present invention disclosed herein is directed to an alternative technique for overcoming the aforenoted difficulties and problems, and for correcting the hue of a displayed video picture as a function of the luminance levels therein.